Optical fiber sensors that use Fabry-Perot cavities to detect pressure and/or temperature are very sensitive and compact. Typical optical fiber sensors include a cavity formed by a diaphragm or a cavity assembly on the end of an optical fiber. Light transmitted through the fiber reflects off both the end of the fiber and the diaphragm, creating a signal that varies with the cavity length, which changes with temperature and pressure. Usually, the thinner the diaphragm, the more sensitive the cavity. The uniformity of the diaphragm thickness is also important; if the thickness varies too much, then the sensor may produce unpredictable, unrepeatable measurements.
Optical fiber sensors are small and their geometrical flexibility make them easy to connect to current tools and to use in small or restricted spaces. Optical fiber sensors generally are immune to electromagnetic interference, inert to chemical erosion, and insensitive to thermal variations. In addition, optical fiber sensors can survive in high-pressure, high-temperature cure cycle environments, such as those encountered during structure fabrication, system integration, and daily use.
Micro Fabry-Perot cavities may be fabricated using a combination of surface micromachining technology. (See, e.g., Y. Kim and D. P. Neikirk, “Micromachined Fabry-Perot Cavity Pressure Transducer,” IEEE Phot. Technol. Letters, 7: 1471-1473, 1995, incorporated herein by reference in its entirety.) Micromachined cavities exhibit good repeatability and can be made with well-controlled cavity lengths. Micromachined cavities typically have limited cavity lengths, however, due to the nature of the surface micromachining process; reported lengths are 0.6 μm and 1.6 μm, which may not be useful in every application. In addition, fluctuations in temperature can cause different layers of the micromachined cavities to change cavity length (i.e., thickness or shape of the diaphragm), affecting measurement accuracy. Also, fabrication techniques may result in a lack of uniformity inside or between wafers. Micromachined cavities are also difficult to integrate with optical fiber due to the complexity of the cavity (sensor) assemblies.
Bulk-micromachining cavities on wafers can make both fabrication and the resulting device structures simpler. Though fiber sensors with bulk-micromachined cavities may be easier to assemble and more accurate than surface-micromachined cavities, the uniformity of the cavity length in bulk-micromachined cavities is difficult to control because the wafer thickness can vary and the fabrication process can be non-uniform.
Moreover, fiber sensors with micromachined cavities must be assembled by fixing the cavities to the fibers with glue or epoxy, which requires extremely precise alignment. The cavities are usually larger than the optical fibers, so the sensors tend to be fragile. In addition, the glue or epoxy may not hold at high temperatures or pressures; worse, the glue or epoxy may shrink or expand at a different rate than the surrounding material, degrading the sensor's temperature stability.
To avoid problems with glue or epoxy, the sensor head can be bonded directly to the optical fiber with one of a variety of bonding techniques. For example, a sensor can be made by coating a thin film with a polyimide spacer, then bonding the surface coated with polyimide spacer to the end face of an optical fiber to form a cavity. Unfortunately, polyimide's properties depend greatly on temperature, so polyimide-based sensors are not suitable for use in harsh environments.
Sensors made using laser fusion bonding use ferrules instead of polyimide to connect thin fused-silica diaphragms to optical fibers. The diaphragms are connected to a ferrule, which is then bonded to an optical fiber. Ferrule-based devices have wide working temperature ranges plus outstanding temperature stability. They are bulky, however, and the diaphragms tend to be too thick for applications that require high sensitivity. In addition, the cavity length is difficult to control during mass production.
Splicing is a well-known way to bond together pairs of optical fibers that can also be used to bond a fused silica diaphragm to the end of an optical fiber. Splice-bonded sensors are compact—the silica diaphragm has a diameter equal to that of the optical fiber—and exhibit outstanding thermal performance. With splicing, though, it is still difficult to control the thickness of the diaphragm and the cavity length. Typically, spliced sensors are made by splicing a fiber with a flat end to a fiber that has been partially etched away. One of the fibers is cut away with a cleaver, leaving a relatively thick (e.g., 3-6 μm) diaphragm that can be wet-etched, if desired, to create a thinner diaphragm. Unfortunately, cleaving tends to leave a non-uniform diaphragm, and the wet etching used to reduce the diaphragm thickness also tends to result in non-uniform thickness. Moreover, it is extremely difficult to splice the bonded fiber thin enough to form a diaphragm that is less than about 3 μm thick. Repeatability is also a problem because spliced sensors are fabricated individually.
Anodic bonding is another well-known bonding technique that can be used to bond silicon to glass. Usually, the fiber end face is etched using photolithography, then a thin silicon film is bonded directly to the etched end face. Anodic bonding has the following drawbacks: (1) the fiber tip must be large enough to handle during photolithography, e.g., 200 μm or more in diameter; (2) the fiber tip must be coated with silver during photolithography, increasing the fabrication cost; (3) anodic bonding between silicon and glass fibers requires that the fiber be specially doped with alkali ions, such as Na+, La+, or K+; (4) anodically bonded sensors have poor sensitivity because the silicon diaphragm is usually 3-10 μm thick; and (5) the diaphragm and the fiber are made of different materials, so thermal stability is still an issue.
Therefore, there is a need for a miniature optical fiber pressure sensor and a method for making such a sensor that overcomes or substantially reduces the above-mentioned problems.